This invention relates to apparatus for perforating wells, and more particularly to a shaped charge apparatus of the semi-expendable type.
Semi-expendable perforating devices typically comprise an elongated support along which are fixed radially directed encapsulated shaped charges. The assembly is lowered into a borehole to the depth at which it is desired to perforate the borehole casing and, after firing, the support is brought back up to the surface, with any pieces of the charge cases which have remained attached to the support. The parts of the charge cases broken free by the explosion constitute debris which remains in the well bore, but this amount of debris is limited thanks to the recovery of the support.
The supports used are often in the form of an elongated strip having attachment holes designed to receive the charges. Such devices are described for example in U.S. Pat. No. 2,756,677 (J. J. McCullough). For certain applications (for example, the preparation of a cased producing zone for the formation of a gravel pack), it is desirable to provide perforations of large diameter and in large number. These high charge density devices involve many constraints which have hitherto not been solved by the prior art.
Supports have been devised with a spirally twisted strip to obtain charges directed along several radial directions. The support described in the abovementioned patent does not make it possible to fit a high charge density because of its very design and as a result of its lack of ruggedness. Such a support is twisted over its length after the attachment of the charges. As the charge attachment holes are also deformed by the twisting, it is possible that the charges will not be held with sufficient strength. In addition, if the known devices are used for well casings of different diameters, the same performance quality is not obtained everywhere. In well casings of large diameter, only the charges which bear against the casing exhibit good performance. The other charges, whose front faces are relatively far from the casing wall, lose a considerable part of their effectiveness. In perforating devices intended for the preparation of gravel packs, it is particularly important to obtain perforations of large diameter (2 cm, for example) spaced as regularly as possible in all directions. With prior art devices which, for example, can provide a shot density of as much as four holes per foot, it would be possible to obtain twice that density by lowering two of these devices to the same depth, but there is no known method for inserting them to obtain perforations with a regular distribution.
Another drawback of known semi-expendable perforating apparatus is the large amount of debris left in the well after the shaped charges are exploded. In fact, the explosion breaks almost all the charge cases into fragments, leaving on the support only the part of these cases actually fixed in the support. This drawback is particularly important for apparatus having a high density of charges.
It is thus desirable to have perforating devices capable of receiving a high charge density and offering excellent ruggedness but with a low manufacturing cost. This low cost is important because, even though the supports can generally be reused, they do become damaged or deformed from time to time, and then must be replaced.